1. Field of the Invention
The invention relates generally to subterranean well tools such as inflatable packers, bridge plugs or the like, which are set through the introduction of an actuating fluid into an expandable elastomeric bladder and, more particularly, to an apparatus and method that utilize a multi-stage piston with multiple operating surfaces in contact with hydrostatic well pressure for maintaining a relatively uniform fluid pressure in the bladder when the tool is subjected to thermal variants after setting.
2. Description of Problems
It is known among those skilled in the use of these types of inflatable devices that they are subject to changes in inflation pressure when the temperature of the inflation fluid varies from its initial inflation temperature. Typically, an increase in fluid temperature results in increased inflation pressures, and a decrease results in decreased inflation pressures. An increase in inflation pressure can make the tool susceptible to burst failure. A decrease in inflation pressure can diminish anchoring between the tool and the well bore to a point where the tool is not able to provide its intended anchoring function. In both instances, significant changes in temperature in the inflation fluid can result in compromised tool performance and possible tool failure. These failures can result in significant monetary loss and possible catastrophe.
The magnitude of temperature change needed to adversely affect the performance of an inflatable tool depends upon a number of parameters, such as, for example (1) the expansion ratio of the inflation element, (2) the relative stiffness of the steel structure of the inflation element compared with the compressibility and thermal expansion coefficient of the inflation fluid, (3) the relative stiffness of the casing and/or formation compared with the compressibility and thermal expansion coefficient of the inflation fluid, and (4) the inelastic properties of the elastomeric components in the inflation element. There are other factors of lesser significance known to those skilled in the relevant art.
Regardless of the specific values of the aforementioned parameters, conventional inflatable tools cannot tolerate positive or negative temperature changes greater than about 10-15 F..degree. (5.6-8.3 C..degree.) from the initial temperature at the end of their inflation cycle. If the temperature of the inflation fluid varies by more than this amount, the tool is subjected to excessive inflation pressures or insufficient inflation pressures, which could result in tool performance problems of the nature described above.
In addition, cycling the inflation fluid temperature within a .+-.15 F..degree. of the initial temperature upon expansion can cause stress cycling in the steel structure of the inflation element and in the bladder. There is the potential for a serious problem when the inflation element survives routine thermal cycling for a finite period oftime, during which cyclic damage in the tool accumulates. In such a case, failure can occur at some time after the rig has departed from the well site. Thus, an inflatable tool can provide short term functional performance during low magnitudes of thermal cycling. However, cumulative damage phenomena can occur in steel structures and/or elastomeric components and eventually cause device failure.
A time delayed failure can be more costly and possibly more catastrophic than one which occurs within a short time after the initial setting of the tool. Replacement of the failed device would entail performing a second project about equal in size and expense to the first service operation, instead of the case of a short-lived tool which would fail before the rig is broken down and moved off the site. Operations of this type can cost in excess of one hundred thousand dollars, and as high as several millions of dollars.
There are many operations in the oil and gas industry that successfully use pressure isolation devices which routinely encounter substantial thermal excursions and substantial magnitudes of combined positive and negative thermal cycling. Typically, inflatable devices are excluded as candidates for such projects. Typical projects are listed below.
large volume stimulation projects, n PA1 selective zone treatment projects, n PA1 large volume cement squeeze projects, n PA1 production packer service in oil and/or gas wells experiencing cooling from Joules-Thompson expansion and cooling of gases, n,c PA1 production packer service in oil and/or gas wells experiencing heating from deeper produced fluids, p,c PA1 conversion of a producing well to an injection well and temporary isolation between perforation intervals, n,c PA1 huff/puff steam injection methods for producing viscous oil formations, p,c
[n=these operations typically result in a large negative thermal excursion (cooling) in the pressure isolation device.]
[p=these operations typically result in a large positive thermal excursion (heating) in the pressure isolation device.]
[c=these projects typically repeated multiple thermal cycling in the pressure isolation device over long periods of time.]
The first five project categories are very common in the industry. Thousands of them are performed per year. The bottom two categories are relatively infrequent with respect to world wide activities.
If conventional packers and bridge plugs are not able to provide service for a given well configuration, because they are not able to pass through restrictions and subsequently set in casing, it is common to use a rig to pull tubing and perform a costly work-over project. The use of thru-tubing inflatable devices provides well known benefits and versatility to the oil and gas industry. Their lack of service worthiness for operations that include thermal cycling and thermal excursions exclude them from a substantial portion of the remedial service sector. An invention that would eliminate the deleterious effects of routine thermal excursions and thermal cycling, would eliminate the aforementioned problems, augment the benefits and versatility of inflatable devices and provide substantial cost savings to operators in the industry.
3. Description of the Prior Art
Subterranean well tools, such as conventional packers, bridge plugs, tubing hangers, and the like, are well known to those skilled in the art and may be set or activated a number of ways, such as mechanical, hydraulic, pneumatic, or the like. Many of such devices contain sealing mechanisms which expand radially outwardly upon the introduction of a substantially incompressible actuating fluid for setting the device in the well to provide a seal in the annular area of the well between the exterior of the device and the internal diameter of well casing, if the well is cased, other tubular conduit, or along the wall of open borehole, as the case may be.
Frequently, the seal is established subsequent to the setting of such device in the well and will be adversely effected by temperature variances of the device or in the vicinity of the device. Such temperature variances can cause expansion or contraction of the sealing mechanism, thus jeopardizing the sealing and even anchoring integrity of the device over time. For example, such devices are typically utilized in well stimulation jobs in which an acidic composition is injected into the formation or zone adjacent a well packer or bridge plug. As the stimulation fluid is injected into the zone, the temperature of the device and the well bore in the vicinity of the formation will be reduced.
If, for example, the well tool utilizes a sealing mechanism that includes an inflatable elastomeric bladder, the temperature of the actuating fluid utilized to inflate the bladder and retain same in set position in the well is affected by the temperature reduction during the stimulation job, causing a reduction of pressure within the interior of the bladder, fluid chambers and communicating passageways within the tool. This reduction in pressure, in turn, causes the bladder to contract from the initial setting position. In more dramatic situations, anchoring of the device in the well bore can be lost and the differential pressures across the device can cause "corkscrewing" of the coiled tubing or work string, resulting in project failure, expensive solution of the corkscrew problem and substantial operational risks.
On the other hand, the same inflatable tool is also adversely affected by an increase in device temperature during certain types of secondary and tertiary injection techniques utilizing, for example, the injection of steam. As the steam is injected into the zone of the well immediate the set packer or well plug, the zone and accompanying devices, including tubing, quickly become exposed to the increased temperature. Some prior art devices containing inflatable packer components have been known to have the inflatable bladder element actually rupture, due to exposure to increased pressure within the bladder and interconnected chambers and passageways as steam flows through the device and is injected into the well zone.
In U.S. Pat. No. 4,655,292, entitled "Steam Injection Packer Actuator and Method," a device is shown and disclosed, which addresses the problems associated with the prior art by providing a mechanism incorporating a compressible fluid, such as nitrogen gas. The fluid is used to accommodate an increase in temperature during steam injection and other operations for preventing the packer mechanism from rupturing as a result of exposure to enhance pressures resulting from the increase of temperature of inflation fluid and device components as stream flows through the device.
PCT application, Ser. No. WO/98/36152, the description and drawings of which are incorporated herein as though fully set forth, describes a thermal compensating apparatus that utilizes hydrostatic well pressure for maintaining a relatively constant pressure in the bladder of an inflatable tool. The apparatus has a piston with a pair of opposed surfaces, which are respectively in contact with the fluid used to actuate the tool and the surrounding well fluid below the tool. The surface in contact with the well bore fluid is proportionately larger in surface area than the surface in contact with the actuating fluid, at a ratio of about 1.4:1 to 1.8:1. Relatively constant hydrostatic well pressure bears on the larger of the surfaces. Referencing off of the hydrostatic well pressure, the piston moves in response to any change in volume and concomitant pressure in the actuating fluid due to temperature changes in the vicinity of the tool, for maintaining a substantially constant pressure in the actuating fluid.
However, the apparatus in the PCT application is not suitable for smaller-diameter thru-tubing tools such as, for example, tools 21/8" in diameter which are commonly run through 27/8" tubes that have internal diameter restrictions of 25/16" and set in a 7" casing. These thru-tubing tools are inflated to high expansion ratios and therefore are filled with a substantial volume of actuation fluid. The volume of actuation fluid is exceptionally high when compared to the area and volume sweeping capacity of the pressure maintaining piston in a single state device having an intensification ratio of 1.4:1 to 1.8:1. These types of tools do not have a large enough diameter to provide a differential surface area on the respective fluid contact surfaces that is great enough to compensate for temperature variances greater than 10-15 F..degree.. Because temperature variances in excess of 20 F..degree. are not uncommon, there is a need for an apparatus that utilizes hydrostatic well pressure for maintaining a relatively constant pressure in small diameter thru-tubing tools in service operations that experience substantial variances in tool temperature while in service.
The present invention addresses these problems associated with the prior art devices, and maintains a relatively constant inflation pressure even when the device experiences single and/or multiple thermal excursions of substantial magnitude. The invention operates to abate the adverse effects of any combination of heating and cooling, both quasi-static and dynamic cycling.